The present invention relates to data storage, and more particularly, to a data storage read channel architecture with reiterated digital front-end control and dropout mitigation.
The Information Storage Industry Consortium (INSIC) International Magnetic Tape Storage Roadmap reveals that areal recording densities are likely to exceed 50 Gbit/in2 prior to 2023. Accordingly, future tape systems will need to operate their read channels at signal-to-noise ratio values that will be significantly (about two dBs) lower than those available in today's tape drives. Under these conditions, ensuring reliable read channel operation at error rates of 1×10−17 or lower becomes an extremely challenging task.
Traditional approaches for achieving more reliable read channel operations, such as reducing the jitter noise in the timing recovery loop, detecting the presence of dropout events and attempting to mitigate their effects, or protecting data by more powerful Error Correction Coding (ECC) schemes, etc., also suffer from problems. These solutions fall short of providing the amount of robustness that is needed for reliable operations under the severely degraded conditions that may often prevail in future-generation tape storage systems.
Major difficulties arise in connection with signal timing recovery, typically, because the reliability of the data estimates driving the timing recovery loop at the projected low signal-to-noise ratio (SNR) operating condition is usually very poor. As a result, the readback signal is sampled at time instants that may deviate significantly from the ideal sampling instants. This in turn not only leads to a degradation of the read channel performance but also leads to the occurrence of cycle slips, which are temporary loss-of-lock events experienced by the phase-locked loop. Also, cycle slips may lead to severe performance degradation that may not even be remedied by error-correction coding, typically. In particular, occurrence of cycle slips may obliterate all the advantages associated with using powerful capacity-approaching coding, such as low-density parity-check (LDPC) coding.
Traditional approaches for combating cycle slips fall short of providing the amount of robustness that is needed for reliable operations under the severely degraded conditions that are likely to prevail in future-generation tape storage systems.
In addition, major difficulties also arise due to the presence of dropout events (dropouts). Dropouts may be attributed mainly to basefilm/back-coat formulation and asperities. Measurements have shown that dropout occurrence frequency increases with linear density and shrinking of the reader width. As a result, dropout events may lead to error bursts at the detector output. Maintaining symbol clock during a dropout is a major challenge, typically. Also, the severe performance degradation caused by dropouts may often not even be remedied by error-correction coding, typically. In particular, dropout events may obliterate all the advantages associated with using powerful capacity-approaching coding.